The reaction of diketene acetals with alcohols to produce poly (ortho ester)s was disclosed in U.S. Pat. No. 4,304,767. Known bioerodible poly(ortho ester)s, such as those described in U.S. Pat. No. 4,549,010, are comprised of linear polymer chains joined to one another by non-covalent intermolecular forces such as Van der Waal's dispersive forces and hydrogen bonding. These poly(ortho-ester)s are thermoplastic, i.e., their individual polymer chains slip by one another ("flow") if subjected to stress. This results in an irreversible deformation of the elastomers and accounts for their lack of resiliency when subjected to prolonged stress. Such characteristics are particularly disadvantageous in applications such as a gastric retention platform wherein the polymers, after having been subjected to stress during prolonged storage, are expected to behave in a resilient fashion.
A gastric retention platform, as disclosed in U.S. Pat. Nos. 4,735,804 and 4,758,436, is a drug delivery device comprised of a resilient polymer combined with an active ingredient. The platform is maintained in a first compressed state from the time of manufacture until ingestion. Subsequent to ingestion, the platform expands to a second relaxed state, thereby releasing the active ingredient to an environment of use (such as the stomach). The prolonged stress and strain suffered during storage limits the resilience of linear, thermoplastic polymer platforms and impedes their ability to expand to the second relaxed state subsequent to ingestion.